Traditionally flexible pipe is utilised to transport production fluids, such as oil and/or gas and/or water and aqueous media (when associated with oil or gas production), from one location to another. Flexible pipe is particularly useful in connecting a sub-sea location (which may be deep underwater, say 1000 meters or more) to a sea level location. The pipe may have an internal diameter of typically up to about 0.6 meters. Flexible pipe is generally formed as an assembly of a flexible pipe body and one or more end fittings. The pipe body is typically formed as a combination of layered materials that form a pressure-containing conduit. The pipe structure allows large deflections without causing bending stresses which would impair the pipe's functionality over its lifetime. The pipe body is generally built up as a combined structure including metallic and polymer layers.
Unbonded flexible pipe has been used for deep water (less than 3,300 feet (about 1,000 meters)) and ultra deep water (greater than 3,300 feet) developments. The increasing demand for oil is causing exploration to occur at greater and greater depths where environmental factors are more extreme. For example in such deep and ultra-deep water environments the cold ocean floor temperature increases the risk of production fluids cooling to a temperature that may lead to pipe blockage. Increased depths also increase the pressure associated with the environment in which the flexible pipe must operate. As a result the need for high levels of performance from the layers of the flexible pipe body is increased.
Flexible pipe may also be used for shallow water applications (for example less than around 500 meters depth) or even for shore (overland) applications. The skilled person appreciates that different constructional considerations apply for flexible pipe intended for these different environments.
Flexible pipes typically comprise an inner fluid retaining layer (also known as the internal pressure sheath) which defines a bore within which fluid transmitted through the pipe is contained. Some types of flexible pipe, known as “rough bore” pipe, include a carcass layer internally of the inner fluid retaining layer. Flexible pipe without a carcass layer is generally referred to as “smooth bore” pipe.
The carcass layer may usefully be of an interlocked construction that can be used as the innermost layer to prevent, totally or partially, collapse of the internal pressure sheath due to external pressure, tensile armour pressure and/or mechanical crushing loads, or in the event of pipe decompression. In known examples of flexible pipe, the carcass layer may comprise helically wound and interlocked steel strip material.
In rough bore pipes the first layer over the carcass layer (which may be the inner fluid retaining layer) is usually a polymer layer, typically an extruded polymer layer. This polymer layer tends, at least partially, to conform to the outer shape of the carcass layer. Such conformation introduces irregularities into the inner surface profile of the polymer layer as it follows the external form of the carcass layer. The irregularities may be in the form of local projections and valleys such as nubs or cusps. These irregularities are undesirable as they can act as areas of stress concentration. This is particularly undesirable in the case of the inner fluid retaining layer, since this layer is a critical pressure containing layer of the flexible pipe body.
In smooth bore and rough bore flexible pipes one or more polymer layers may lie adjacent a reinforcement layer, such as an armour layer. One example of such a layer is a polymer barrier layer internally adjacent a metallic pressure armour layer. Such polymer layers may be subjected to quite severe non-uniform, highly localised strain deriving from the non-uniformity of the inner surface profile of the overlying armour layer. This is because the armour layer is usually formed from interlocking wires, and there are gaps, troughs, valleys or the like between adjacent windings. The underlying polymer layer may thus tend to deform and creep into the gaps when under pressure.
In flexible pipes polymer layers are typically formed by extrusion.
Most polymers will have a certain maximum allowable strain above which the risk of damage to the material is much greater. It is therefore desirable to avoid use conditions where such maximum allowable strain is exceeded, or to construct the flexible pipe body in such a way that the maximum allowable strain is not approached, even in arduous use conditions.
By way of example, one situation in which the maximum allowable strain of a polymer layer could potentially be approached is in a factory acceptance test procedure. In accordance with industry regulations, all flexible pipe structures must undergo a factory acceptance test (FAT) prior to sale. This involves pressurising a pipe bore with a fluid such as water at 1.5 times the usual pressure of use. The fluid is thus a pressurising medium.
The application of internal pressure (i.e. pressure from within the bore) to the pipe may produce radial expansion in all layers and a polymer layer may (as indicated above) thus undergo deformation and may tend to creep into the gaps of an overlying armour layer. At high pressures (about 8000 psi/55 MPa or more), the resultant strain distribution within the polymer layer can be highly localised at the areas around the gaps, and the polymer material may deform by cavitation rather than plastic flow. This can in turn result in the formation of microcrazing or microcracking on the radially inner surface of the polymer layer. During any subsequent loading (such as the loading experienced during normal use in transporting production fluids) this microcrazing may then extend to form longer and/or deeper cracks throughout the polymer layer. This can increase the risk of failure of the polymer layer and may ultimately lead to loss of pressure containment, having an adverse effect on the lifetime of a flexible pipe.
It is known for flexible pipe body to be provided with one or more wear layers. The provision of such wear layers may be desirable and useful, for example to prevent or mitigate wear of a barrier layer caused through relative movement of the barrier layer and the carcass layer. A wear layer may be further advantageous in providing a smoother and more uniform surface onto which a barrier layer can successfully be extruded, in contrast to the outer surface of the carcass layer which, because of the construction of the carcass layer, is often not smooth and may contain recurring ridges and depressions and the like. A wear layer provided between a barrier layer and an outer (with respect to the barrier layer) pressure armour layer can be advantageous in accommodating some degree of creep between the barrier layer and the pressure armour layer, which can be useful in avoiding crazing of the barrier layer.
However, the provision of one or more wear layers can have attendant disadvantages. For example, it is advantageous in general to seek to reduce the number of layers of a flexible pipe body in order to improve manufacturing efficiencies and to reduce weight and material cost (while maintaining effectiveness of the pipe body in use throughout the service life of the pipe—typically 25 years). It is further known that occasionally in service the carcass layer and barrier layer of the flexible pipe body may internally collapse due to a pressure build up between the wear layer and the barrier layer, especially when the bore pressure (of the fluid being conveyed in the flexible pipe) is released. This is an issue uniquely associated with flexible pipe body structures incorporating multi-layer extrusions.